This invention is related to the printing of data supplied to a plurality of remote printers from a central processing unit over a common transmission channel.
Prior art communication systems, for example controlling the environment within a building or fire and security conditions within a building, having a central panel for providing alarm indications or indications of the status of various points within the building, originally provided a separate wire running from the panel to each point which was being monitored. In today's large buildings, however, the costs of wiring in this manner can be prohibitive. Therefore, the prior art replaced these plurality of wires by a single transmission channel running from the central processing unit to the remote points being monitored.
The central processing unit is capable of scanning the status of the various remote points and controlling various operations at the remote points by addressing in turn each remote point and requiring the remote points to take some action. The action may be to report back its present status, such as temperature, pressure or humidity, or to perform some operation such as starting or stopping a fan, opening a damper or whatever. By using a single transmission channel, the cost of installation of these monitoring systems is greatly reduced.
Such communication systems must perform a variety of services other than scanning and performing operations at remote points. Indications of the status of the remote points must be given and recorded which function is often performed by printers. In a typical system the printer will print out any changes in status of the remote points. It is also desirable for the printer to print logs of the status or occurrences at the remote pointers. In the present invention, the printer has access to the central processor to request the transmission of certain data so that the printer can print it. The printer used in the present invention is a rotary drum printer.
Such rotary drum printers have a printing drum with characters arranged in a plurality of columns and rows. Paper is fed between this drum and a plurality of firing hammers, each hammer associated with a corresponding column of characters and the printing operation is accomplished by firing a hammer which strikes the paper to the character leaving imprinted on the paper the character on the drum associated with that hammer.
Data to be printed which is transmitted by a central processor typically comprises a plurality of bits and the bits are divided into groups of four or five, each group representing a character to be printed. The data is transmitted, usually, a line at a time. A plurality of bits, comprising a plurality of characters, are supplied to the printer which stores them and then pulls out the data bits, a character at a time, for comparison with the present drum position of the printer. Each character to be printed corresponds to a particular character in a particular column on the drum. If the drum presents its first row of characters to the hammers only certain stored characters will correspond to the characters in that row. Thus, only those hammers relating to the characters in the row which match the stored characters will be fired.
The drum rotates to a new row after the hammers have been fired at the first row. The stored characters are compared with the new drum position and new corresponding hammers are fired. This operation continues until all rows have been compared with the stored characters. As the last row is compared, the entire line will have been printed and the printer can print a new line of data from the central processor.
In the prior art, when data is to be printed at more than one printer, the first printer must be addressed by the central processing unit and the central processing unit must then send out the data; the central processing unit must then address the second printer and send out the data to be printed again and so on. This operation will continue until all printers, which have been selected by the central processing unit, have printed out the data. As will be noted, this operation requires the data to be sent out once for each printer which is to print the data.
The present invention, however, allows the central processing unit to send out a plurality of addresses each designating a selected printer to print the data and then send out the data in only one transmission for printing by the selected printers. In the present system, each word sent over the transmission line is sent twice, the first during the first half of the transmission and the second during the second half of transmission. Thus, the first half can be compared to the second half, bit by bit, to detect any possible errors in transmission. Therefore, "transmission" is defined here as either sending out word messages only once, or more than once for purposes other than sending out data in the prior art method of transmitting data a number of times depending upon how many printers are to print the data.